Interim Considerations for Phased Implementation of COVID-19 Vaccination and Sub-Prioritization Among Recommended Populations

Summary of Recent Changes

Revisions made on March 2, 2021

  • Addition of considerations for location accessibility, communicating vaccine information and scheduling appointments for certain populations
  • Updates to considerations for vaccination in congregate living settings
  • Updates on implementation and dosing considerations for Pfizer, Moderna and Janssen COVID-19 vaccines
Key Points

The Advisory Committee on Immunization Practices (ACIP) recommends that, when supplies of COVID-19 vaccine are limited, vaccination should be offered in a phased approach. Phase 1a includes healthcare personnel and long-term care facility residents. Phase 1b includes persons ≥75 years of age and frontline essential workers. Phase 1c includes persons 65-74 years of age, persons 16-64 years of age with high-risk medical conditions, and other essential workers as defined in each jurisdiction. The following guidance is meant to aid public health programs and immunization partners in planning for the transition between and sub-prioritization of populations in Phase 1b and 1c of COVID-19 vaccination implementation.

These considerations apply in the context of limited vaccine supply and will be updated as needed based on changes in vaccine supply, COVID-19 epidemiology, or other factors. CDC published a companion guide to assist state, tribal, local, or territorial immunization programs and other immunization partners in planning for vaccination of these populations.


The following guidance is meant to aid public health programs and immunization partners in planning and implementing COVID-19 vaccination programs.

Transitioning Between Vaccination Phases

Considerations for transitioning between phases are needed to ensure expeditious transition from one phase of COVID-19 vaccine allocation to the next (i.e., from Phase 1a to 1b or from Phase 1b to 1c) as vaccine supply increases and exceeds demand within specific populations or geographic locations in a given phase, or when low demand puts vaccine doses at risk for going unused. It is not necessary to vaccinate all individuals in one phase before initiating the next phase; phases should overlap to optimize distribution of existing vaccine supply.

CDC offers the following considerations for transitioning from one phase to the next, recognizing that these considerations will likely need to be adapted to the local, state, tribal, or territorial context and may change over time. Decisions regarding transition from one phase to the next may be based on individual factors such as COVID-19 epidemiology, vaccine supply and demand within the jurisdiction. These considerations may also be applied to transitions among groups sub-prioritized within a phase, in conjunction with factors specific to jurisdictional needs.

Once COVID-19 vaccination has been expanded to additional phases, jurisdictions should continue to offer and promote vaccination to all people in earlier phases who have not yet been vaccinated. If vaccination sites within a jurisdiction are moving independently through the phases and are more than one phase apart (e.g., some sites are ready to move to Phase 1c while others are still in Phase 1a), jurisdictions should assess vaccine supply, demand, and equitability of vaccine distribution, and consider re-distribution of vaccine or other ways to expand vaccination access, demand, and capacity, and address vaccine hesitancy within the jurisdiction. For example, jurisdictions may need to reassess and increase efforts to support people in making their vaccination decisions.

Considerations for Transitioning Between Phases

Decisions about transitioning to subsequent phases should depend on COVID-19 epidemiology, vaccine supply and demand, equitable vaccine distribution, and local, state, tribal, or territorial context. These suggested considerations are meant to help inform assessments of the current phase and decisions on when to transition to the next phase.

Jurisdictions may consider expanding vaccine availability to priority groups in the next phase in several situations, including, for example:

  • When demand in the current phase appears to have been met (e.g., appointments for vaccination are <80% filled for several days)
  • When supply of authorized vaccine increases substantially (e.g., more vaccine doses are available than are necessary to complete vaccination of people in the current phase)
  • When most people in the current phase are vaccinated (e.g., when a specified percentage of the target population in a phase has been vaccinated)
    • Strong efforts should be made to increase demand and access in each priority group. In cases where coverage targets are not met, jurisdictions may consider expanding to the next phase in order to ensure that vaccine doses are used.
  • When vaccine supply within a certain location is in danger of going unused unless vaccination is expanded to people in the next phase

Sub-prioritization of Frontline and Other Essential Workers

Frontline essential workers and other essential workers have been recommended by CDC’s Advisory Committee on Immunization Practices (ACIP) to receive vaccination in Phases 1b and 1c, respectively.

ACIP defines frontline essential workers as the subset of essential workers likely to be at greatest risk for work-related exposure to SARS-CoV-2, the virus that causes COVID-19, because their work-related duties must be performed onsite and their duties involve being in close proximity (<6 feet) to the public or coworkers. Further information on the essential workforce can be found in the Interim List of Categories of Essential Workers Mapped to Standardized Industry Codes and Titles.

Frontline essential workers recommended by ACIP to receive vaccination in Phase 1b are: fire service, law enforcement, corrections workers, food and agricultural workers, U.S. Postal Service workers, manufacturing workers, grocery store workers, public transit workers, and those who work in the education sector (teachers and support staff members) as well as daycare workers. Essential workers recommended by ACIP to receive vaccination in Phase 1c include people who work in: transportation and logistics, water and wastewater, food service, shelter and housing, finance, information technology, communications, energy, legal services, media, public safety, and public health.

In settings where the vaccine supply is insufficient to vaccinate all workers in a given phase (e.g., frontline essential workers in Phase 1b, other essential workers in Phase 1c), state, local, tribal, and territorial jurisdictions will need to decide how to sub-prioritize groups of people for COVID-19 vaccination. As jurisdictions make these complicated decisions, they will need to keep in mind that the intent of the recommendations is to ensure equitable access to vaccination among people at highest risk for severe disease and those who perform essential services and functions. Sub-prioritization among groups of essential workers may differ by jurisdiction, given the potential variability in the composition of non-healthcare essential workers required to ensure continuity of functions that are critical to public health and safety, security, and the economy at jurisdictional and national levels.

Considerations for Sub-prioritization

Within a given phase, jurisdictions may consider sub-prioritization of the following groups of workers, based on potential exposure risk:

  • Groups of workers who are the most critical to maintaining core societal functions
  • Groups of workers with unavoidable higher risk of exposure because of their inability to perform work duties remotely and those who work in inherently high-risk situations such as those involving crowding or close contact with coworkers or members of the public (e.g., people in meatpacking plants who might be working in close quarters, personal care assistants who are providing care for individuals in group homes, or 911 telecommunication centers)
    • Considerations may be informed by the length of time workers are exposed to each other and the public, and the number of contacts they have during a typical workday.
  • Groups of workers in settings where high rates of transmission and outbreaks have been documented to occur with greater frequency (e.g., people in meatpacking plants, correctional and detention facility workers)
  • Groups of workers who have been disproportionately affected by COVID-19 (e.g., people in groups impacted by health disparities and inequities such as migrant farm workers)

Considerations for Other Populations in Phase 1b and 1c

Sub-prioritization may also be necessary among other groups included in Phase 1b and 1c. Jurisdictions should consider the unique needs of residents, such as people with disabilities or cognitive decline (and their caretakers), as well as those with limited access to technology, when evaluating vaccination location accessibility, communicating vaccine information, and scheduling appointments.


Communities that have been disproportionately affected by COVID-19 and have a younger population distribution, such as American Indian and Alaska Native communities, may choose to adopt a different lower age limit for vaccination prioritization appropriate to state, local, tribal, or territorial epidemiology.

High-risk underlying medical conditions

Through review of the medical literature, CDC has identified a list of conditions that increase an individual’s risk of severe COVID-19. People with underlying medical conditions not identified as a high-risk condition should consult with a healthcare provider about their personal risk factors. As COVID-19 vaccines become more widely available in providers’ offices and pharmacies, healthcare providers may use clinical judgement to determine an individual patient’s risk of severe COVID-19. Verification of eligibility for an underlying condition should not hamper throughput at large vaccination clinics.

Congregate living settings

Increased rates of transmission have been observed in congregate living settings. Therefore, jurisdictions may choose to prioritize vaccination of persons in these settings based on local, state, tribal, or territorial epidemiology. Congregate living facilities include settings such as correctional or detention facilities, homeless shelters, group homes, or employer-provided shared housing units. People living and working in these settings may have challenges with social distancing or other mitigation measures to prevent the spread of COVID-19. Jurisdictions may choose to vaccinate persons who reside at congregate living facilities at the same time as the staff, because of their shared increased risk of disease.

COVID-19 Vaccines

Currently three COVID-19 vaccines have been authorized under an Emergency Use Authorization issued by the U.S. Food and Drug Administration and recommended by the Advisory Committee on Immunization Practices. Pfizer and Moderna mRNA COVID-19 vaccines both consist of a 2-dose series. Johnson & Johnson’s Janssen COVID-19 vaccine requires only 1 dose. Any of the currently authorized COVID-19 vaccines can be used when indicated during any given phase; ACIP does not state a product preference. Additional information on available COVID-19 vaccines can be found in the Interim Clinical Considerations for Use of COVID-19 Vaccines Currently Authorized in the United States.

The 1 dose Johnson & Johnson/Janssen COVID-19 vaccine can be transported and stored at 3-8oC. These product characteristics may be amenable to use in mobile clinics or sites that do not have freezer capacity necessary for storing the mRNA COVID-19 vaccines. The single-dose vaccine might be desirable for people who want or need to complete their immunization schedule quickly or who might have difficulty returning for a second dose. In addition, a single dose vaccine may be desirable for mobile populations or populations with high turnover, such as homeless shelters or correctional facilities.


The Advisory Committee on Immunization Practices’ Updated Interim Recommendation for Allocation of COVID-19 Vaccine — United States, December 2020

Strategies for Populations Recommended to Receive Initial Doses of COVID-19 Vaccine | CDC

Interim List of Categories of Essential Workers Mapped to Standardized Industry Codes and Titles

Page last reviewed: March 2, 2021